Wavefront distortion produced by transmission through abdominal wall,
breast, and chest wall has been measured in a two-dimensional aperture and the
abdominal wall measurements have been compared to distortion simulated using
sound speed and density determined from stained cross sections. The results show
that wavefront distortion varies with tissue type, scattering contributes
significantly to distortion, and arrival time and energy level fluctuations can
be large compared to increments in current beamformers. Focus degradation and
compensation has also been investigated by reconstructing the time history of a
virtual point source without compensation, with time-shift compensation in the
aperture, and with backpropagation followed by time-shift compensation.
Backpropagation followed by time-shift compensation was found to be more
effective than time-shift compensation in the aperture. A comparison of
wavefront distortion and compensation in one- and two-dimensional apertures has
shown that distortion is smoothed increasingly by one-dimensional apertures with
larger elevation dimensions and that one-dimensional compensation becomes less
effective than two-dimensional compensation for larger elevations. Pulse-echo
studies using different f-number transmitters have shown that wavefront
aberration is significantly underestimated with a broad incident beam and
demonstrated that a sharply focused incident beam is necessary for good
estimation of time shifts and for effective compensation.